Press rolls, and in particular granite press rolls, are useful in paper making and cardboard manufacturing machines, in addition to having other uses. Generally, the press rolls are held in press roll apparatus or assemblies by steel heads or flanges which are rotatably supported by journals. The mechanical stress and temperature fluctuations which the rolls are placed under in industrial uses can lead to axial cracking, circumferential fracture, or fatigue failure of the granite roll body.
The most catastrophic press roll failures have resulted from axial cracking, which stems basically from thermally induced tension in the stone primarily caused by greater thermal expansion of the steel heads engaging the ends of the granite roll which has a much lower coefficient of thermal expansion than steel. Consequently, expansion of the heads causes circumferential tension stresses in the press rolls. Additionally, the likelihood of catastrophic failure of press rolls increases in modern, high-speed machines in which higher temperatures and centrifugal forces increase tension on the roll.
Vestola, et al., in Granite Rolls--Failures Their Prevention and Substitute Materials, Proceedings of the 1989 Annual Meeting CCPA, Montreal, Canada, B-109 (1989), teach that shaft failure can be avoided by proper design, and that circumferential fracture can be reduced by appropriate dimensioning and pretensioning. However, there remains a need to eliminate the source of the most catastrophic press roll failures--axial cracking caused by circumferential or hoop stress.
Vestola, et al. indicate that granite rolls having shafts fitted tightly into a bore in the roll, or shafts surrounded by concrete grouting, will generally have a high hoop stress at the inner surface of a granite roll when there is a rapid temperature rise in the shaft; such rapid temperature rise could be due to heating of the shaft caused by friction or mechanical malfunction. However, even where the shaft is not fitted tightly, or in a non-grouted roll, Vestola, et al. teach that stress in granite press rolls can not be totally eliminated; this belief is based on the principle that differential thermal expansion of the metallic heads at the roll ends will cause circumferential stresses at the ends of the granite roll. The circumferential stress is believed to be due to the fact that the heads will radially expand faster than the roll ends due to the higher coefficient of thermal expansion of the steel forming the heads than that of the roll material (usually granite) and that there is great friction between the granite roll and the steel heads, so that relative movement (slippage) of the head flanges in contact with the roll end surfaces is not possible.
A prior art solution to the radial expansion problem of press rolls involved compression of a roll having opposed male conical shaped ends matingly engaged with matching conical female surfaces on steel heads. With reference to FIG. 1, one end of such a prior art press roll is illustrated, with it being understood that the opposite end of the press roll is substantially a mirror image thereof. A press roll 2, made of granite or other like suitable material, is axially aligned and concentric about a shaft 4 which passes through bore 6. A centering ring 8 ensures that shaft 4 is radially centered in roll 2 about the axis of rotation. Press roll 2 terminates in tapered conical surface 10.
A compression head 12 is connected relative to shaft 4, and includes an annular ring 14, a portion of which is pressed against the male tapered conical end surface 10 of press roll 2. The inner surface 16 of annular ring 14 is a tapered conical surface identical to and making surface to surface contact with the male tapered conical surface 10 so as to create a smooth and continuous interface Axial force exerted against the heads presses the heads against the tapered conical ends of a press roll, to create radial force against the stone and create radial compression and stress in the press roll.
Compression of the heads prior to assembling the press roll apparatus can be achieved by extending the length of the shaft by heating the shaft relative to the press roll and then cooling of the shaft, which will induce both longitudinal (axial) and radial compression in the press roll. An example of such a method and apparatus is shown by Hill in U.S. Pat. No. 3,737,962. It is also possible to use one or more tie-rods, threadbars, or cables in place of, or in addition to, a central shaft for inducing tension between opposed press roll heads. See Muhle et al., U.S. Pat. No. 4,642,862. Further, a solid roll having opposed male tapered ends can be compressed between two female heads which are compressed by a force external to the roll.
U.S. Pat. No. 4,924,688, to Cutmore, and U.S. Pat. No. 4,991,275, to Adams, Sr., also show roller assemblies having rolls with male tapered ends which are placed in axial compression by hubs or collars having inner surfaces tapered at an angle to match the tapered ends of the roll. All references cited herein are incorporated by reference as if reproduced in full below.
The nature of the compressive forces created in a roll by the prior art methods is more easily understood by referring back to FIG. 1. Longitudinal compression in shaft 4, illustrated by force line 18, is transformed into a smaller longitudinal component 20 and a radial compression component 22 due to the non-rectilinear interface between the female tapered end 10 of press roll 2 and the male tapered or bevelled inner surface 16 of compression head 12. Upon heating of a roll having a male tapered end compressed between opposed female tapered or conical heads, the longitudinal compression component may be reduced due to greater longitudinal expansion of the shaft with respect to the length of the roll; this longitudinal expansion of the shaft may be offset by longitudinal expansion of the head and roll body. However, the head also expands radially outward, and friction between the head and the roll end ma contribute to circumferential stress in the roll.
Further, conical shaped granite roll ends and their complimentary steel heads are difficult to manufacture, especially with any degree of precision; failure to achieve a smooth interface between tapered roll ends and tapered heads can result in asymmetric compression of the roll which can lead to cracking of the roll or otherwise shorten the life of the roll. Further, differential thermal expansion of the steel heads, internal shaft (if used), and centering rings (if used) induce stress in the press roll.
The difficulties in machining tapered press roll ends and in manufacturing complimentary conical roll heads creates the need for a method for inducing radial compression in press rolls having a rectilinear interface between the roll ends and heads or clamping plates. There is also a need for a press roll in which circumferential stress, induced by differential thermal expansion of the roll heads, internal shaft (if used), and centering ring (if used), is substantially eliminated at the operating temperature of the press roll.
Thus, it is a primary object of the present invention to manufacture a press roll apparatus having a rectilinear interface between the ends of the roll and the heads which has no substantial circumferential stress at operating temperature.
It is a further object of the present invention to provide a method of constructing press roll apparatus which have a rectilinear interface between the roll and the heads and which have no substantial circumferential stress at operating temperature.